The present invention relates to an LED lighting device drive circuit and an image display device using the same.
Recently, with increase of the light emission efficiency, the light emitting diode (LED) is employed for a lighting device more and more. Especially the LED is tried to be employed for a backlight arranged at the rear surface of a liquid crystal display (LCD) device. As a display method of the liquid crystal display device, in addition to the TN (twisted nematic) as a mainstream, an IPS (in-plane switching) and an MVA (multi-domain vertical alignment) characterized by a wide visual angle are used. These devices form an image by introducing the light of the lighting device arranged on the rear surface of the display unit into a liquid crystal panel capable of controlling the transmittance of the light. As a light source of the lighting device, in addition to the LED, it is possible to use a cold cathode fluorescent light (CCFL), a hot cathode fluorescent light (HCFL), an organic light emitting diode (OLED), and the like. The LED has a high color purity and can increase the color reproduction range of the liquid crystal display device. Moreover, since no lead is used, it is appropriate for the environment. Furthermore, since the LED has a high-speed response, it is possible to easily perform modulation with a light emission time width, which easily reduces power consumption.
The lighting device as a liquid crystal display device should have a characteristic that the brightness and color will not change. However, the LED has a characteristic that the light emission efficiency changes depending on the temperature and its brightness is changed by the affect of self-heating during ON state. Accordingly, the LED requires a compensation technique so that the brightness and the color will not be changed by the temperature change.
JP-A-2005-310997 discloses a technique for compensating the fluctuation of the brightness of the LED as the time elapses. That is, a photo sensor is divided to detect an emitted light quantity of each LED and feedback is performed on the LED drive condition so as to prevent the brightness change of the LED.
Moreover, JP-A-2004-199896 discloses a technique for arranging a temperature sensor in the vicinity of the LED so as to detect the LED temperature state and performing feedback on the LED drive condition.
Recently, however, a large-screen liquid crystal television exceeding 32 inches is spread. In order to realize this, it is necessary to arrange a plenty of LEDs substantially uniformly in the wide area. For this, it is difficult to compensate the brightness fluctuations of the plenty of LEDs by the techniques disclosed in the aforementioned JP-A-2005-310997 and the JP-A-2004-199896.
For example, in the compensation technique using a photo sensor, when a plenty of light sources are used, the positional relationship between the light source and the photo sensor differs among the light sources and the light quantity received by the photo sensor differs depending on the respective light sources. Accordingly, it is necessary to prepare a compensation table for compensating the positional relationship between the photo sensors and the light sources and estimate the light source emitted from each of the light sources. Such a complicated process in turn increases the circuit cost.
Alternatively, a plenty of photo sensors are required and increase of the number of photo sensors increases the production cost. Moreover, when using light sources of multi-primary colors such as LED for each of the RGB, it is necessary to arrange a color filter for the photo sensor and detect the light of each color, which also increases the production cost.
Moreover, in the compensation technique using the temperature sensor, when a plenty of LEDs are arranged on a large-area surface such as a large-size television, a temperature distribution is caused in the plane due to thermal convection and heat discharge structure. Accordingly, it is actually difficult to detect the temperatures of all the LEDs in the plane by a single temperature sensor and a plenty of temperature sensors are required, which increases the production cost.